Alumina solid solution



Patented Apr. 8, 1947 i ALUMINA SOLID SOLUTION Henry N. Baumann, Jr.,and Raymond C. Benner, Niagara Falls, N. Y., assignors to TheCarborundum Company, Niagara Falls, N. Y., a corporation of DelawareContinuation oi' application Serial No. 506,226,

October 14, 1943. This application October 25, 1944, Serial No. 560,230

24 Claims. (Cl. 10G-65) This invention relates to alumincus materialsand particularly to alumina whose crystalline structure is modified bythe presence of one or and may be produced by fusing bauxite or othermaterial high in alumina under reducing conditions in electric furnacesor by the fusion of alumina of high technical purity as made, forexample by the Bayer process, and beta alumina. The particular abrasivecharacteristics of such forms of alumina will vary somewhat dependingupon the total alumina content, the amount and kind of associatedimpurities and the rate of cooling of the fused product. Under anyconditions, however, such known products have certain limitations asregards uniformity, toughness, microstructure, and the degree to whichtheir physical properties may be altered to llt different abrasiveapplications. i

We have found that when alumina is modified with one or more metaloxides which crystallizeA in the same general form as alpha alumina andthe crystalline dimensions of which are of the same order as those ofalumina, the resulting change in crystal structure furnishes meanswhereby the physical properties of the crystalline alumina may begreatly varied. In the case of cells ci similar dimensions and angles:of the l same order between any two of the three trigonal axes.

more other oxides in the alumina lattice. Table 1 It is an object of ourinvention to produce 5 improved materials of this type by the use of amodifying oxides crystallizing in the same Bumm s@ general form as alphaalumina. It is a further object of our invention to provide a method forrl0 ggg g2g: modifying the surface of alumina granules or lli woafljjjlHexagonl'Rhmbhdmmivisin- 5145 'sansa' articles. EMOL... 5. 42 55, Astill further object 'of our invention is to In Table 1 the edge (a) ofthe unit cell is given in Angstrom units (A), that is 10-8 cm., andalpha (a) is the angle between any two of the three trigonal axes. Ineach instance the unit cells contain the same number of atoms.

While all of the oxides listed in Table 1 will form solid solutions withalumina, the possible extent of such,solution is not the same vwith allof the substances. For example, with chromium sesquioxide (CrzOs) andalpha alumina, similar X-ray patterns are obtained over the entire rangeof compositions, thus showing that a continuous series of solidsolutions is formed.v The only material dierence found in X-ray powderphotograms taken of specimens containing different amounts of A1203 andCrzOa, is in the increased distances between corresponding lines as theamount of chromium oxide is increased. This change is caused by anincrease in size of the unit cell, as atoms of chromium replace atoms ofaluminum in the alpha alumina unit cell and ls a result of the slightlygreater ionic radius of chromium as compared with that of aluminum. Thisdierence in size of the atoms or ions is also responsible for a slightdistortion of the alumina lattice when other oxides are in solidsolution which is believed to be the cause oi the increased hardnessthat we have found in abrasive grain made from those solid solutions andmay aeiacee l as well as manganese sesquioxide (MmOa) enters into solidsolution in alumina to some extent and in so doing produces an eiect ofthe same nature as that produced by the entry of chromium sesquioxideinto solid solution in alumina. Manganese sesquioxide, although notordinarily crystallizing in the hexagonal system, has a radius ratio(the ratio of the radius of the manganese atom to the'radius of theoxygen atom) so close to that of alumina that it can form solidsolutions with the latter. While the amounts of vanadium, i

manganese and iron sesquioxides which will enter into solid solution inalumina are limited, nevertheless the increase in hardness may be evengreater than is obtained with chromium oxide in solid solution becauseof greater distortion of the alumina unit cell.

Unlike chromium and vanadium sesquioxides, manganese and ironsesquioxides when completely fused with alumina (particularly underreducing conditions) tend to dissociate to a lower oxide state of thetypical formula' MeO and to form aluminates rather'than to enter intosolid solution in the alumina. However, when heated with alumina atsintering temperatures or somewhat below iron and manganese oxides enterinto solid solution in the alumina to some extent. It should accordinglybe remembered when manganese and iron oxides are referred to in thisapplication that the use of these oxides to form solid solutions inalumina is only practicable by a process not involving fusion.

Advantages may be taken of the tendency of the above recited oxides toenter into solid solution in alumina by acting upon alumina 4in a numberof ways. Thus, fusions of alumina and either chromium or vanadium oxidesor both may be made with the production of a crystalline prod- .uctwhich consists of a solid solution of the oxide i or which' have beenformed of fused alumina, so

as to produce a layer, case, or thick iilm, of a solid solution of oneor more other metal oxides in an arc type of electric furnace such as isusually used in making fused alumina abrasive grain.

in alumina at the surface of the granules or articles. In many-casesthis may be done without substantially changing the composition of thecenter of the granules or articles. For convenience in reference in thespeciiication and claims, the production of such a layer or case willWhen an electric furnacels used to fuse mixtures of alumina withsolid-solution-forming oxides excessive reducing conditions must beavoided. Accordingly the raw materials must be loW in carbon and arepreferably carbon-free.

However, if small amounts of carbide or free f metal are formed byreduction, they may be eliminated by later heat treatment of theabrasive material. In carrying out such a ilusion method we may allowthe fused material to cool slowly in the furnace, thus forming a largeingot or pig, but ive prefer to' east the molten magma into n pour itout into thin layers.

ng to another method 4of forming alu- Jhd solutions, the alumina, whitealue made by the Bayer process being par- ,.,f suitable, is mixed with asolid-solutionfornnng oxide. Both materials are used in/ilnely powderedform, ne enough, for example, to pass through a screen having 200 meshesper linear inch. To this mixture a temporary binding medium such, forexample, as dextrln may be added, as necessary, and the mixture isformed into blocks or shapes. rFliese are then red to a sufficientlyhigh temperature to produce recrystallization which gives a product witha very dense structure.

In making abrasive grain from the fused prody uct -described above orfrom blocks of the sintered product, theprocess employed follows theusual crushing, screeningand other steps used in commercial practice. Wehave found, however, that the alumina solid solution grain isparticularly amenable to heat treatment and may be given such atreatment to improve its hardness, strength and toughness, In part atleast we believe this is caused by the phenomenon of precipitationhardening.

Case-treatment of alumina granules or articles results, in accordancewith our invention, from heating together such granules or articles andthe metal oxide or oxides, in iinely divided form,

which it is desired to introduce into solid solu-` The heattion at thesurface of the alumina. ing is carried out at temperatures which are notonly far below the melting point of alumina but are also well below thetemperatures necessary to sinter alumina, the solid-solution-formingoxides or mixtures of the alumina and oxide or oxides. In 'fact thetemperatures employed are so low that when granules of alumina have beencase-treated they are very little aggregated and no substantialsintering together ofthe particles of alumina or other oxides isevident. Only gentle milling is required to separate case-treatedgranules to an extent which will permit substantially al1 of themto'passthrough screens of the same mesh size as those usedl in theseparation of the original granules. The surface of case-treated aluminaarticles isy also quite free from adhering particles and only a brushingis needed to remove such particles as do adhere.

being a sectional diagrammatic representation of an article,specifically a sintered alumina nozzle, which has been case-treated inaccordance with our invention. A fuller description of the drawings willbe presented hereafter in connection with a more detailed description ofthe case-treating process.

We have found that the thickness of the case or layer of solid solutionon thesurface of alumina bodies is a function of the time andtemperature of heating the alumina in contact with thesolidsolution-forming oxide or oxides. The duration of heating and thetemperatures employed in obtaining a given thickness of case, however,will depend upon the solid-solution-forming oxide or oxides used. Ofcourse, when articles of alumina,

auch as entered shapes. are being ease-treated, Jimsena solucion isformed in the depression to a' the location and extent o! the ,oase willbe primarily determined by the location and extent of the portion orportions of the articles in contact with the modifying oxide or oxides.

Examples I and II illustrate typical procedure by which alumina granulesmay becase-treated.

vExample I An amount of nely powdered vanadium pentoxide equal to 2% byWeight of the quantity of alumina granules on which it is desired toform a case of AlaOa--VaOa solid solution, is mixed with sufllcientwater to form a thin slurry. After thoroughly mixing the slurry with apredetermined quantity of 36 mesh alumina granules the mixtureis driedand pressed in a refractory container and heated for one hour at 1200 C.After cooling to lroom temperature the product is pressed through a 36mesh screen which serves to break up. the few aggregates which may beformed by the cohesion of the granules. The resulting product is acase-treated granular alumina material of substantially the sameparticle size as the original alumina'particles. The granules of theproduct have a structure like that shown in Figure l of the drawings.

In the product made according to.Example I, the portion of the particlescorresponding to the portion indicated at i2 in Figure 1, is a solidsolution' .of alumina.

Example Ii Where it is desired to form on alumina granules a case ofsolid solution containing a plurality of other metal oxides in alumina,the nely divided oxides, for example V205 and FezOa, may be mixed withWater to form a thin slurry as in Example I. The slurry in appropriateamounts is then added to the calculated quantity of alumina granules.Subsequent treatment of the'mixture is like that in Example I, atemperature of about 1300 C. being used for 11/2 hours. The productobtained is a mass of alumina granules having a solid solution casecontaining both V203 and FeeOs.

The products resulting from the foregoing specific example are granularin nature and have in general a structure like that shown in Figure 1 ofthe drawings. InFigure 1 is shown diagrammatically a section of analumina granule which has been so case-treated that the center of thegranule is substantially unchanged, that is, the solid solution does notextend beyond a shell on the surface of the granule. The casetreatedgranule consists of an inner core li, of substantially unalteredalumina, surrounded by a case or layer l2` of a solid solution of oneo!more other metal oxides in alumina.

The concentration of the metal oxide or oxides in the solid solutioncase or layer of course varies, being greater in the exterior andbecoming progressively less toward the interior of the granule. Thedecreasing concentration of the metal oxide or oxides in solid solutionin the alumina is indicated graphically by the concentration of the dotsin case I2. As will be noted, there is no sharp boundary between thecase or layer of solid solution and the substantially unaltered interiorof the granule since diffusion will notY be, uniform in all directionsin the crystalline granuie. At I 3 is indicated a dimple or depressionin the granule and it will be seen that the vanadium sesquioxide (V203)in' considerable depth.

As previously mentioned the depth lor thickf ness of the solid solutioncase on alumina bodies depends upon the time and temperature of heating.Thus, by varying the time or temperature of heating, or both, thethicknessof the coating may be varied to any desired degree.' Thetemperatures `which we prefer lie between about 1100 C. and 15009 C. Thetime of heating will course vary with the temperature used, but ingeneral at the preferred temperatures mentioned above; a heating periodof 1/2 to 2 hours is sumcient.v

Control of the depth or thickness-of the case as well as its compositionmay also be obtained by varying the amounts of solid-solution-form ingoxides used. In general we have found that about l to 5% of asolid-solution-forming oxide is satisfactory and care should be taken toavoid an excess of oxide or oxide-forming material since if too much isused the undissolved re= mainder will contaminate and in some cases ad`here to the case-treated bodies.

It is not always possible to so treat granules of alumina that theinterior of the granule is entirely unaltered. Thus, where granules oismall size are used, it is usually the case that at least some which arein more intimate con-a tact with the solid-solution-forming oxide oroxides will have a certain amount of the other oxide or oxides in solidsolution even in the interior of the granules. Where the aluminagranules are very small the diloulties of proper regulation are suchthat generally the whole granule will be converted to solid solution.

Figure 2 of the drawings illustrates diagrammatically a sectional Viewof an alumina granule, case-treatment of which has been carried outunder such conditions that the central portion of the granule has beensomewhat altered by the presence of the solid-solution-iorming oxideaithough the amount of alteration present at and near the center of thegranule is very much less than that at the surface. In some cases thesolid-solution-forming oxide may be present at the center in barelydetectable amount. As will be seen, the concentration of dotsrepresenting graphically the concentration of solid solution decreasesmarkedly from the outer portion of the granule indicated generally at 2ito the center of the granule indicated at 2t.

The following example describes the casetreatment of an alumina article,to impart increased toughness, hardness and wear-resistance to thesurface.

Example III A nozzle for a sand-blasting apparatus formed by a knownsintering procedure from line alumina powder may have the wear receivingportion thereof hardened and toughened byapplying to the interior of thenozzle a fairly thin coating of a slurry of a solid-solution-formingoxide (such as V205) in water and heating the nozzle for a period offrom 1/2 to 1 hour at approximately 1200 C. After this treatment it willbe found that a case has been formed on a surface of the nozzle wherethe solid-solution-forming oxide slurry had been applied, thecase beinga solid solution of the oxide in alumina. i

When the entire article is likely to be subwith the slurry ofsolid-solution-forming oxide or forming oxide in very finely dividedform and heated in contact therewith. It will, of course, be obviousthat the usefulness of this treatment is not limited to sand-blastnozzles but that the treatment may be applied to any slntered aluminaarticle in which the improved properties obtained by case-treatment aredesirable. In Figure 3 is shown a diagrammatic sectional view of asand-blast nozzle 3| which has been case-treated in accordance with ourinvention. The nozzle represented in Figure 3 has been casetreated overits entire surface and the con'centration of solid solution indicatedgenerally at 32 at and near the surface is shown graphically by theconcentration of dots.

The methodof Example III is particularly applicable to the improvementof sintered alumina articles which are subject in use to wear orabrasion such as, for example, besides sand-blastingnozzles, pistons,valves, thread guides and extru-y sion devices. It is also possible tocase-treat and thus increase the toughness of sintered alumina articlessubject to high mechanical stress such as drawing dies, machining andcutting tools and mounts therefor, drills, and the like with highlydesirable results.

In a manner somewhat analogous to the casetreatment methods describedabove finely divided alumina (which, as has been pointed out above, isdiflicult if not impossible to treat so as to obtain only a surface caseof solid solution) may be so treated as to convert the entire body ofthe alumina particles to a solid solution and then be sintered inaccordance with well-known or desired procedures 'to form hard, tougharticles composed of an alumina solid solution.

While we do not Wish to be bound thereby we believe that the probablemechanism of the casetreatment is as follows:

When the alumina articles or granules are `coated with 'metal oxide andheated to a relatively high temperature, atoms of the. metal or metals,the oxide or oxides of which are used, migrate across the boundarybetween the oxide and alumina and replace aluminum atoms in the crystallattice of the alumina. This action is progressive, first taking placeat the surface, and

continuing with sustained heating, the solid sof lution being formed inlower concentration nearer and nearer the center of the granules orarticles. As the interchange goes on, there is of course'a tendency forthe other metal atoms to become evenly distributed and hence upon lon'gcontinued heating or in a shorter time with particles of small size theproduct obtained will be converted to a homogeneous solid solution ofthe othermetal oxide in alumina. v

Whatever the correct theory of the formation of the solid solution caseon the alumina bodies, there is no room for doubt that such a layer ofsolid solution exists since Debye-Scherrer X-ray .-photograrns ofpowdered samples of case-treated aluminamade in accordance with ourprocess show deilnite shifts in the pattern lines from the normalalumina pattern. Shifts of this type can be explained only by thepresence of solid solutions. Examination of our case-treated aluminaunder the petrographic microscope reveals a color change of the aluminaadjacent the surface. This change indicates the absorption of othermetal oxides. Additional evidence is found in the fact that refractiveindex determinations of case-treated alumina show an index differing.

from that yof untreated alumina.

The solid-solution-formlng oxides with which it is desired to modify thealumina either in form'- ing masses of solid solutions or incase-treating articles or granules of alumina` may be supplied in anumber of ways. Of course, the pure oxides CrzOs, Vz, MnzOa and FezOsmay be used but ln many cases it will be found more convenient to useother and more readily obtainable or cheaper sources of materials. Wehave .found that many other sources of these oxides may be usedsatisfactorily. Thus, oxides of other compositions,

such as V205 instead of V203, or salts which will i break downto'solid-solution-forming oxides under the conditions of the process maybe used. It is also possible to use oxides or compositions which containconsiderable impurities. For example, chromite may be used as a sourceof chromium oxide even when containing, as an impurity, 40% or moreA1203, and ores, such for example, as vanadium ore containing no morethan 10% V205, may be employed.

In general, purer oxides or compositions will be used when formingsolidl solutions by sinterlng or when the oxides are to be used forcase-treating alumina while the more impure materials may besatisfactorily used when` forming solid solu.

tions by fusion. e

When ores or other oxide sources are use d the residual material in theore or impurities will form a matrix material associated with thealumina, metal-oxide solid solution.` This is particularly noticeablewhen the fusions are made in' small sized Pieces by a casting process orwhen sntering is employed. Where matrix material is present it should ingeneral not exceed about 10% of the composition of the aluminametaloxide solid solution since in greater amounts the continuity of theproduct may be interrupted to an objectionable extent.

As previously pointed out chromium oxide and alumina form a continuousseries of solid solutions whereas the other metal oxides mentioned enterinto solid solution to a limited extent only. Consequently where anexcess of an oxide, even a pure oxide, is used above that amount whichwill form a solid solution with alumina, the excess oxide willconstitute a matrix material in a manner similar to the matrix formed byimpurities. While of perhaps secondary importance to the factfthat wehave found a Way of commercially varying the properties of aluminaabrasives by presence of included or interstitial matter, that is,matrix material, produces a further improvement in some instances. As anexample, when a fusion, instead of being allowed to cool in a mass andthus form large, crystals, is poured out in thin layers or to formshapes having small areas of cross section such as small blocks, itcools quickly, and crystal growth is inhibited. Under such conditionsthe matrix material is well dispersed in the cast mass and it may beobserved petrographically as included and interstitial impuritiesasso-,- ciated with fine crystalline alumina metal-oxide solid solution.The presence of matrix-forming materials is chiefly useful when thegrain is heat treated by roasting as a later step in its preparation foruse as abrasive grit, the heat treatment causing the grains to becometougher, a desirable property in certain abrasive operations.

Where the alumina metal-oxide solid solutions are to be produced bysintering still another source of the metal-oxide may be used, that isthe finely divided metal itself. Under, oxidizing asiatica t Qconditions the metal will be transformed to the oxide and form a solidsolution.

It is of course obvious that since by other metal-` oxides entering intothe unit cell of crystalline alpha alumina the physical structure andconsequently the physical properties of th'e crystal are fundamentallyaltered the abrasive properties of the alumina metal-oxide solidsolution crystals are different from the alpha alumina.

In the abrasive art it is now well-recognized that the grinding,polishing or other working by abrasive processes of different materialsrequires for best results a fitting of the abrasive to the material forthe particular process and that as a result a wide variety of abrasivemedia is required. We have found that the changing of the crystallinecharacter of alumina by the introductionpf oxides which form solidsolutions with alumina is a useful tool for producing a large number ofvarieties of abrasive grain with different microstructures, fractures,toughness, bonding properties, and other physical properties. The rangeof varieties will be better appreciated when it is remembered that,besides the solid solutions in alumina of CrzOs, V203, FezOs, and MnzOa,solid solutions in alumina of mixtures of one or more of V203, FezOa,and MnzOa with Cr203, mixtures of one or both of MnzOa and FezOs withV203, and mixtures ,of MnzOs and FezOs may be employed.

Abrasive grain formed from alumina metaloxide solid solutions isapplicableto a number of abrasive uses in both bonded and coatedabrasives. In general, alumina grain with only small amounts of addedsolid-solution-forming oxides is more suitable for abrasive articlessuch as grinding wheels with ceramic or vitried bonds, wheels of thistype showing particularly good strength in speed tests and theadditional hardness of the abrasive over normal alumina abrasive beingan important factor. Such grain has also' the sharpness and strengthrequired for coated abrasives such as are used, for example; inWoodworking and metal grinding having either glue or resin adhesives.Abrasive grain made from heat-treated fused cast material with higheramounts of added solid-solution-forming oxides, and sometimes matrixmaterial, appears preferable for use in making articles with resinbonds. We do not, however, limit ourselves to any particular uses forthe improved aluminous abrasives,

til

since our invention broadly covers the distinctly new class of aluminaabrasive compositions disclosed herein.

` Alumina abrasive grain which has been casetreated by the process vofour invention possesses certain differences over the abrasive grainl inwhich the solid solution is uniformily distributed. In many instancesthese differences are advan tageous since, due .to the fact that thegrain surface has properties different from those of the center or coreof the grain, the fracture of the grain will be more irregular, andsince greatery variation of the properties of the treated grain,

toughness' exceeding that of untreated alumina grain or even aluminagrainA which has been heated to the same temperature' as that used inour process. The following table shws the comparative crushing strengthsof various alumina grain samples:

` These tests were made on single 16 grit grains on a testing machine inwhich the grain was placed between two tables and crushed by pre`ssure.'I'he results of this test are believed to indicate to a considerableextent the relative toughness of the abrasive grain and they show thatthe case-treatment has considerably' increased the toughness.

The amountsof solid-solution-forming oxides employed may vary over awide range. Of course, when making a mass of an alumina solid solutionthe presence of excess solid-solution-forming oxide is oftenunobjectionabl'e since except in the case of chromiumwhich forms solidsolutions in all' proportions with aluminum, the excess oxide willappear in the product as matrix material. We have found, however, that`a desirable degree of hardness and toughness can be obtained with anamount of solid-solutioneforming oxide of from about 1A to about 10 or20%, depending upon the solubility of the oxide. In cases where nomatrix material is desired or `where it is desired not to contaminatethe final product, as in i generally satisfactory. With Very largealumina granules or with shaped alumina articles it is frequently amatter of convenience to use somewhatv more solid-solution-forming oxideor oxidesrsince the time necessary for case-treatment will besomewhatdecreas'ed.

In accordance with our invention physical properties of aluminousgrainmay be varied in one or more of the following ways:

(1) We may vary the kind of material going into solid solution.

(2) We may vary the amount of material in solid 1 solution. (3) We mayvary the kind and amount of matrix material.

(4) We may vary the location of the material in l solid solution. (5) Wemay, by heat treatment, vary the hardness and toughness.

Where percentages are given in the specication and the claims it wm beunderstood that percentages by weight are meant unless it is otherwisespecified.

Where in the appended claims we refer to solidsolution-forming oxides orto CrzOa, FezOa. V203 and MmOs We mean to include, wherethe contextpermits, materials for example, salts, other vdicated. Where the with,comprising a solid solution of V20:

oxides andthe metals themselves, which under the conditions of theprocess form these oxides or other oxides of the named metalswhich enterinto solid solution in alumina. By the unmodiiled term "alumina we meanany form of alumina except Where a contrary intention is inexpressionessentially crystalline alumina is used in the claims it is meant bythis expression to denote crystalline alumina containing only the smallamounts of impurities present in the alumina body before case-treatment.'I'his application is a continuation of our copending application,Serial No. 506,226, led October 14, 1943. Other uses of our inventionwill no doubt b e appreciated by those skilled in the art andaccordingly we do not wish to be limited except by the scope of theaccompanying claims.

We claim:V

1. As a new article of manufacture, abrasive grain comprising aluminacontaining in -solid solution V203.

2. As an abrasive grain', alumina which contains in solid solution V203and with which is associated as inclusions and interstitially a matrixmaterial.

3. A modified alumina body consisting of an interior portion which isessentially crystalline A1202 and a surrounding layer, integraltherewith, comprising a solid solution of at least one oxide selectedfrom the group consisting of CR203, V202. Fe20s and Mn203 in crystallinealumina.

4. A modified alumina body consisting of an interior portion which isessentially crystalline VA121011 and a surrounding layer, integraltherewith, comprising a solid solution of Cr20a in crystalline alumina.

5. A modified alumina body consisting of an interiorportion which isessentially crystalline A1203 and a surrounding layer, integraltherecrystalline alumina. v

6. A modified alumina body consisting of an interior portion which isessentially crystalline A1202 and a surrounding layer, integraltherewith, comprising a solid solution' of Mn20a in crystalline alumina.

7. A case-treated alumina body consisting of a core and a oase,surrounding and integral with said core, said case comprising a solidsolution of at least one metal oxide selected from the group consistingof Cr202, V203, Fe20a and Mn20a in crystalline alumina and said corebeing essentially crystalline alumina free from the solid solution. v

8. A body consisting principally of" crystalline alumina having in solidsolution therein at least one oxide selected from the group consistingof Cr20a, V203, Fe20a and Mn203, the concentration of the solid solutionforming oxide decreasing from the surface of the body to a barelydetectable concentration at the center thereof.

9. A body consisting principally of crystalline alumina having in solidsolution therein Cr20a,

the concentration of the latter decreasing from concentration at thecenter thereof.

11. A body consisting principally of crystalline alumina having in solidsolution therein Mn203, the concentration of the latter decreasing from12 the surface of the body to a barely detectable con.. centration atthe center thereof.

12. A case-treated crystalline alumina granule consisting of a coreportion and a surrounding case. integral therewith, comprising a solidsolution of at least one metal oxide selected from the group consistingof Cr20a, V203, Fe203, MmOs in crystalline alumina, the amount of metaloxide in solid solution ranging from the smallest useful amount up toabout 5%.

13. A case-treated crystalline alumina granule consisting of a coreportion and a surrounding case, integral therewith, comprising a solidsolution of Cr20s in crystalline alumina, the ,amount of Cr20s in solidsolution ranging from the smallest useful amount up to about 5%.

14. A case-treated crystalline alumina granule consistingof a coreportion and a surrounding case, integral therewith, comprising a solidsolution of V202 in crystalline alumina, the amount of V203 in solidsolution ranging from the smallest useful amount up to about 5%.

15. A case-treated crystalline alumina granule consisting of a coreportion and a surrounding case, integral therewith, comprising a. solidsolution of Mn202 in crystalline alumina, the amount of Mn203 in solidsolution ranging from the smallest useful amount upto about 5%.

16. I'he method of treating alumina bodies which comprises heating acrystalline alumina body with at least a portion of the `surface thereofin contact with at least one finely divided metal oxide selected fromthe group consisting of Cr203, V203, Fe202 and Mn203 ata temperaturebelow the sintering points of the alumina and other metal oxides for aperiod of time suilicient to cause the metal oxide or oxides to enterinto solid solution in the crystalline alumina.

17. The method of treating alumina bodies which comprises heatingv acrystalline alumina' perature below the sintering points of thealuminaand Cr203 for a period of time suicient to cause y the Cr203 toenterinto solid solution in the crystalline alumina.

1 8. The method of treating alumina bodies which comprises heating acrystalline alumina body with at least a portion of the surface thereofin contact with finely divided V203 at a temperature below the sinteringpoints of the alumina and V203 for a period oftime suiiicient to causethe V203 to enter into solid solution in the crystalline alumina.

1.9. The method of treating alumina bodies which comprises heating acrystalline alumina. body with at least a portion of the surface thereoflin contact with iinely divided Mn203 at a temperature below thesintering points of the alumina and Mn203 for a period of timesufficient to cause the Mn203 to enter into solid solution in thecrystalline alumina.

20. The method of case-treating granular crystalline alumina whichcomprises heating a mixture of granular crystalline alumina and at leastone finely divided metal oxide selected from the group consisting ofCr20a, V202, Fe20a and Mn203 at a temperature below the sintering pointsof the alumina and other metal oxides for a period of time sufcient toform a case of a solid solution of crystalline alumina and other metaloxide or oxides around a substantially unchanged core of crystallinealumina.

21. The method of case-treating granular crystalline alumina whichcomprises heating a mixture of granular crystalline alumina and finelydivided CrzOs at a temperature below the sintering points of the aluminaand4 CrzOs for a period of time sufficient to form a. case of a solidsolution of crystalline alumina and CrzOs around a substantiallyunchanged core of crystalline alumina.

22. The method of case-treating granular crystalline alumina whichcomprises heating a mixture of granular crystalline alumina and iinelydivided V203 at a temperature below the sintering points of the aluminaandVzOs for a period of time sumcient to form a case of a solid solutionof crystalline alumina and V203 4around a substantially unchanged coreof crystalline alumina.

23. The method of case-treating granular crystalline alumina whichcomprises heating a mixture of granular crystalline alumina and iinelydivided MnzOa at a temperature below the sintering points of the aluminaand MnzOa for a period j of crystalline alumina and other metal oxide oroxides around a substantially unchanged core of crystalline alumina, thetotal amount of other metal oxide or oxides ranging from the smallestuseful amount up to aboutI 5%.

HENRY N. BAUMANN, JR.

ARAYMOND C. BENNER.

REFERENCES CITED The following references are of record inthe le of thispatent:

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